To reduce wind flow resistance and drag on a semi-trailer truck, trailer, truck, semitrailer, or other vehicle, aerodynamic devices may be used to redirect and control the flow of air passing around the vehicle. During operation of a semi-trailer truck, air around the semi-trailer truck may flow between the tractor unit and the trailer and impart a drag force to the trailer. Aerodynamic devices are designed to control the air flowing into the gap formed between the tractor unit and the trailer. Such reduction on the drag of the vehicle may operate to conserve fossil fuels as well as other sources of vehicle drive power for hybrid vehicles, battery-operated vehicles, and alternative fuel-based vehicles, for example.